This invention relates to the polymerization of fluoropolymers into porous substrates. The fluoropolymer/substrate network that is present on the surface of the substrate and is also deposited into the substrate at appreciable depths. Depending upon the proportion of fluoropolymer relative to substrate, the fluoropolymer may provide a protective coating for the substrate and/or the substrate may improve the physical properties of the fluoropolymer.
Porous materials have a host of uses. Common uses for leather and porous polyurethane are to produce clothing and furniture. Common uses for wood include use as a building material and for the production of furniture. Polyimide compositions are known to have unique performance characteristics, which make them suitable for uses in the form of bushings, seals, electrical insulators, compressor vanes, brake linings, and others as described in U.S. Pat. No. 5,789,523. Para-oriented aromatic polyamides (para-aramids) are used to make fiber substrates that are useful for wear resistant application.
All of the porous materials described may degrade and decay over time by staining, wetting, warping, tearing or wearing. It is desirable to treat porous materials to improve resistance to wear, tear, creep, decay, and degradation by wetting, staining and warping, and to improve durability while maintaining the appearance of the materials.
For many years, textiles have been chemically treated to improve water and oil repellency. Different applications are commercially available to protect different kinds of substrates from oil and water staining. For example, Scotchgard(copyright) brand protector for fabrics sold by the 3M Company, and Teflon(copyright) Fabric Protector sold by E. I. du Pont de Nemours and Company, are available to consumers for use with textiles and fabrics. The use of granular fluoro-compounds is also discussed in Japanese Patent 05318413. The invention involves a method whereby a raw wood material is impregnated with a fluorinated microparticles having a diameter of 5 microns and a compound which changes to insoluble cured resin.
Other references include the treatment of microporous materials with fluoroacrylate to achieve permanent water and oil repellency. For example, U.S. Pat. No. 5,156,780 teaches a method for treating microporous substrates to achieve water and oil repellency while maintaining porosity. In the ""780 method, the substrates are impregnated with a solution of monomer in a carrier solvent. The carrier solvent is first substantially removed from the substrate for the express purpose of leaving the monomer as a thin conformal coating on all internal and external substrate surfaces. In this manner, the monomer is converted to polymer and the polymer does not block the pores or restrict flow in subsequent use as a filtration membrane.
If enough fluoromonomer is polymerized into a porous structure, a point is reached at which there is more fluoropolymer than substrate and the composition can be considered a filled fluoropolymer. Fluoropolymers such as PTFE are commonly filled with substances such as glass fibers, graphite, asbestos, and powdered metals (Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Volume 11, John Wiley and Sons, New York, pages 626 and 630). The filler is generally added for the purpose of improving some property of the fluoropolymer, such as creep or hardness.
Most often, filled fluoropolymers are made by physically mixing the fluoropolymer with the filler or by coagulating an aqueous fluoropolymer emulsion on the filler, but such methods have their problems. Adhesion of fluoropolymer to filler can be quite poor, particularly if the fluoropolymer does not wet the filler and penetrate its pores and finer surface features. Fluoropolymer melts can be very stiff making mixing/dispersion poor and nonuniform. Mechanical mixing can degrade some fillers, for example by breaking fine fibers. It is desirable to polymerize fluoromonomer onto the surface and into the pores of a substrate to achieve intimate fluoropolymer/substrate interpenetration and dispersion with minimal mechanical stress.
Disclosed in this invention is a process for preparing a fluoropolymer/substrate composition, comprising:
in the case of gaseous fluoromonomer
(a) contacting a porous substrate with a solution comprising an initiator dissolved in a suitable solvent;
(b) exposing said substrate and said initiator to gaseous fluoromonomer under polymerization temperature and pressure conditions wherein the fluoromonomer polymerizes into said substrate;
or in the case of liquid fluoromonomer
(a) preparing a solution comprising initiator and liquid fluoromonomer;
(b) contacting a porous substrate with said solution; and
(c) polymerizing the liquid fluoromonomer under polymerization temperature and pressure conditions wherein the fluoromonomer polymerizes into said substrate, optionally in the presence of gaseous fluoromonomer.
Also disclosed is a composition of matter made by a process for preparing a fluoropolymer/substrate composition, comprising:
in the case of gaseous fluoromonomer
(a) contacting a porous substrate with a solution comprising an initiator dissolved in a suitable solvent;
(b) exposing said substrate and said initiator to gaseous fluoromonomer under polymerization temperature and pressure conditions wherein the fluoromonomer polymerizes into said substrate;
or in the case of liquid fluoromonomer
(a) preparing a solution comprising initiator and liquid fluoromonomer;
(b) contacting a porous substrate with said solution; and
(c) polymerizing the liquid fluoromonomer under polymerization temperature and pressure conditions wherein the fluoromonomer polymerizes into said substrate optionally in the presence of gaseous fluoromonomer.
A further disclosure of the present invention is a composition of matter, comprising: a substrate having a surface wherein the substrate further comprises polymerized fluoropolymer, and wherein the substrate is an open pore structure having interconnecting pores throughout said substrate, and wherein fluoropolymer is present within and on the surface of said composition at a level from about 0.1 percent to about 300 percent of the weight of said substrate.
Also disclosed is the use of these compositions as filler materials for other polymers.
The present invention discloses a fluoropolymer/substrate composition. The presence of fluoropolymer in the composition provides a protective material for the substrate and may also add aesthetic qualities to the substrate. A further advantage of the fluoropolymer/substrate composition is that the physical properties of the fluoropolymer are improved.
Also disclosed in the present invention is a method for preparing intimately interpenetrated fluoropolymer/substrate compositions that improve the functional lifetime and/or the appearance of any or all the components. The method disclosed for making the fluoropolymer/substrate composition leaves the initiator and the initiator carrier solvent in the substrate during polymerization and uses undiluted monomer or, in the preferred embodiment, gaseous monomer, to penetrate and block all pores to the greatest depth possible. In the present invention, the polymerized fluoromonomer partially or completely fills and blocks the pores of the substrate.
Coating the surface and blocking the pores of a substrate with fluoropolymer prevents or slows degradation by wetting and penetration of the substrate by agents such as water, acids, bases, foodstuffs, and cosmetics, thereby preventing staining, warping, and unwanted chemical or physical property changes in the substrate. As a case in point, the Ultrasuede(copyright)/PTFE composition of Example 8 below wets less readily than untreated Ultrasuede(trademark). Coating the surface and blocking the pores of a substrate with fluoropolymer can also slow mechanical degradation by such means as abrasion, creep, or tearing. As a case in point, the polyimide/PTFE composition of Example 2A abraded 8xc3x97 more slowly than untreated polyimide.
Going further, once the volume of polymerized fluoropolymer exceeds that of the substrate or once the fluoropolymer/substrate network has been blended into pure fluoropolymer, the substrate can then be considered as dispersed in the fluoropolymer for the purpose of modifying fluoropolymer properties. These compositions are commonly referred to as xe2x80x9cfilled fluoropolymer.xe2x80x9d For example, intimately interpenetrated porous polyimide or aramid particulates can be added to poly(tetrafluoroethylene) to potentially decrease PTFE creep. In a process disclosed in the present invention, the fluoromonomer is polymerized both on the surfaces and into the pores of a substrate to achieve intimate fluorpolymer/susbtrate interpenetration and dispersion. The filled fluoropolymer is prepared with minimal mechanical stress. This process reduces degradation, and thereby, offers a solution to the problem of degradation that occurs with mechanical mixing.
The invention involves a process for the in situ polymerization of fluoromonomer into substrates. Polymerization temperatures range from about 0xc2x0 C. to about 300xc2x0 C., preferably from about 0xc2x0 C. to about 100xc2x0 C., most preferably from about 5xc2x0 C. to about 30xc2x0 C. For those substrates that retain their rigid pore structures at high temperatures and do not thermally decompose, polymerizations can be run at temperatures up to about 300xc2x0 C.
The process of the present invention uses fluoromonomer in either the gaseous or liquid state. Gaseous monomers include tetrafluoroethylene (TFE), trifluoroethylene, vinylidene fluoride, chlorotrifluoroethylene, hexafluoroisobutylene and perfluoro methyl vinyl ether. Liquid monomers include 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole (PDD), perfluoro (2-methylene-4-methyl-1,3-dioxolane (PMD) and perfluoro propyl vinyl ether. These monomers may be homopolymerized or copolymerized to make compositions known to those skilled in the art. Examples include tetrafluoroethylene homopolymer and tetrafluoroethylene/4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole copolymer.
By xe2x80x9cporous substratexe2x80x9d is meant any solid material penetrated throughout with interconnecting pores of a size such as to allow absorption of liquid initiator solution and monomer. The porous substrates can take any form including microscopic particulates, microscopic fibers, coarse particulates, pulp, fibrids, chunks, blocks, uncompressed, partially or fully compressed parts, sheets, films, membranes, and coatings. Porous substrates are not meant to include materials such as cloth where the only mechanism of fluoropolymer entrainment is gross entrapment between separate fibers rather than subsurface penetration into a substrate""s pores. This process works with any porous substrate that does not inhibit fluoromonomer polymerization. Substrates not inhibiting polymerization include wood, wood by-products such as paper, p-aramid fibers, molded polyimide parts, porous polyurethane and leather. Whether a substrate will inhibit polymerization must be determined empirically substrate by substrate and may vary for the same substrate, depending upon prior finishing and treatment.
The present invention also provides a fluoropolymer/substrate composition wherein the substrates are open structures with interconnecting pores throughout their bulk and the level of fluoropolymer in the fluoropolymer/substrate composition is about 0.1% to about 300% of the weight of the substrate. Substrates useful in this invention include wood, paper, leather, porous polyurethane, and aramids and polyimides that have been precipitated as porous particulates or porous fibers and then left wet, dried, or molded only so far as to preserve enough porosity for subsequent penetration by fluoromonomer and initiator. Preferred substrates are porous aramid, polyimide particulates and polyimide parts.
When a preferred substrate is used, the porous aramid or polyimide is immersed for about 1 minute in a 0.1 to 0.2 M solution of hexafluoropropylene oxide dimer peroxide (DP) 1
CF3CF2CF2OCF(CF3)(Cxe2x95x90O)OO(Cxe2x95x90O)CF(CF3OCF2CF2CF3 1, DP 
in CF3CFHCFHCF2CF3 solvent. The excess solvent is filtered off or is drained from the aramid or polyimide, and the still damp polymer placed in a container with 1 atmosphere pressure of tetrafluoroethylene gas until the substrate has gained preferably 5 to 20% of its weight by polymerization of the tetrafluoroethylene to poly(tetrafluoroethylene).
The preferred aramids are poly(p-phenylene terephthalamide) (hereinafter xe2x80x9cPPD-Txe2x80x9d) fibers and poly(m-phenylene isophthalamide)(hereinafter xe2x80x9cMPD-Ixe2x80x9d) in the form of fiber, particles, pulp or fibrids, that are dried, or never-dried. Examples of preferred aramids are poly(p-phenylene terephthalamide) fibers sold by the DuPont Company under the tradename xe2x80x9cKevlar(copyright)xe2x80x9d, and poly(m-phenylene isophthalamide) sold by the DuPont Company under the tradename Nomex(copyright).
A xe2x80x9cnever-dried aramidxe2x80x9d means an aramid coagulated from a solution by contact with a non-solvent (usually an aqueous bath of some sort, such as water or an aqueous solution). When contacted with the non-solvent, the polymer coagulates and most of the solvent is removed from the aramid. The aramid has an open sponge-like structure, which usually contains about 150-200% by weight of the aramid of non-solvent (again, usually water). It is this open sponge-like structure, which has imbibed the non-solvent, which is referred to herein as xe2x80x9cnever-dried aramidxe2x80x9d.
By PPD-T is meant the homopolymer resulting from mole-for-mole polymerization of p-phenylenediamine and terephthaloyl chloride and, also, copolymers resulting from incorporation of small amounts of other aromatic diamine with the p-phenylene diamine and of small amounts of other aromatic diacid chloride with the terephthaloyl chloride. Examples of other acceptable aromatic diamines include m-phenylene diamine, 4,4xe2x80x2-diphenyldiamine, 3,3xe2x80x2-diphenyldiamine, 3,4xe2x80x2-diphenyldiamine, 4,4xe2x80x2-oxydiphenyldiamine, 3,3xe2x80x2-oxydiphenyldiamine, 3,4xe2x80x2-oxydiphenyldiamine, 4,4xe2x80x2-sulfonyldiphenyldiamine, 3,3xe2x80x2-sulfonyldiphenyldiamine, 3,4xe2x80x2-sulfonyldiphenyldiamine, and the like. Examples of other acceptable aromatic diacid chlorides include 2,6-naphthalene-dicarboxylic acid chloride, isophthaloyl chloride, 4,4xe2x80x2-oxydibenzoyl chloride, 3,3xe2x80x2-oxydibenzoyl chloride, 3,4xe2x80x2-oxydibenzoyl chloride, 4,4xe2x80x2-sulfonyldibenzoyl chloride, 3,3xe2x80x2-sulfonyldibenzoyl chloride, 3,4xe2x80x2-sulfonyldibenzoyl chloride, 4,4xe2x80x2-dibenzoyl chloride, 3,3xe2x80x2-dibenzoyl chloride, 3,4xe2x80x2-dibenzoyl chloride, and the like. As a general rule, other aromatic diamines and other aromatic diacid chlorides can be used in amounts up to as much as about 10 mole percent of the p-phenylene diamine or the terephthaloyl chloride, or perhaps slightly higher, provided only the other diamines and diacid chlorides have no reactive groups which interfere with the polymerization reaction.
By MPD-I is meant the homopolymer resulting from mole-for-mole polymerization of m-phenylenediamine and isophthaloyl chloride and, also, copolymers resulting from incorporation of small amounts of other aromatic diamine with the m-phenylene diamine and of small amounts of other aromatic diacid chloride with the isophthaloyl chloride. Examples of other acceptable aromatic diamines include p-phenylene diamine, 4,4xe2x80x2-diphenyldiamine, 3,3xe2x80x2-diphenyldiamine, 3,4xe2x80x2-diphenyldiamine, 4,4xe2x80x2-oxydiphenyldiamine, 3,3xe2x80x2-oxydiphenyldiamine, 3,4xe2x80x2-oxydiphenyldiamine, 4,4xe2x80x2-sulfonyldiphenyldiamine, 3,3xe2x80x2-sulfonyldiphenyldiamine, 3,4xe2x80x2-sulfonyldiphenyldiamine, and the like. Examples of other acceptable aromatic diacid chlorides include 2,6-naphthalene-dicarboxylic acid chloride, terephthaloyl chloride, 4,4xe2x80x2-oxydibenzoyl chloride, 3,3xe2x80x2-oxydibenzoyl chloride, 3,4xe2x80x2-oxydibenzoyl chloride, 4,4xe2x80x2-sulfonyldibenzoyl chloride, 3,3xe2x80x2-sulfonyldibenzoyl chloride, 3,4xe2x80x2-sulfonyldibenzoyl chloride, 4,4xe2x80x2-dibenzoyl chloride, 3,3xe2x80x2-dibenzoyl chloride, 3,4xe2x80x2-dibenzoyl chloride, and the like. As a general rule, other aromatic diamines and other aromatic diacid chlorides can be used in amounts up to as much as about 10 mole percent of the m-phenylene diamine or the isophthaloyl chloride, or perhaps slightly higher, provided only the other diamines and diacid chlorides have no reactive groups which interfere with the polymerization reaction.
The process invention disclosed herein works for most organic initiators commonly used for fluoroolefin polymerizations, including, but not limited to, diacylperoxides, peroxides, azos and peroxydicarbonates. The preferred initiator is DP. DP has a half-life of about 4 hours at 20xc2x0 C. which means that DP lasts long enough for a polymerization run to be set up at room temperature without excessive initiator loss and yet DP still reacts fast enough at room temperature for polymerizations to run to completion fairly quickly. Preferred run times are from about 4 to about 24 hours.
In the preferred embodiment of this invention, the initiator is first synthesized in any solvent that is compatible with fluoroolefin polymerization and the initiator solution then absorbed into the substrate. Suitable solvents comprise chlorofluorocarbons such as Freon(copyright) 113 (CFCl2CF2Cl), hydrofluorocarbons, such as Vertrel(copyright) XF (HFC-43-10mee; 2,3-dihydroperfluoropentane) specialty fluid, perfluorocarbons, such as perfluorohexane, perfluoroethers, such as Fluorinert(copyright) FC-75 sold by 3M Company, perfluoroamines, such as Fluorinert(copyright) FC 40, and perfluorodialkylsulfides, such as CF3CF2CF2CF2SCF2CF2CF2CF2CF3. The preferred solvents for DP are Vertrel(copyright) XF and Freon(copyright) E1(CF3CF2CF20CFHCF3).
In this invention, the preferred initiator solution comprises a solution of hexafluoropropylene oxide dimer peroxide [DP] in Vertrel(copyright) XF (CF3CFHCFHCF2CF3). It is further preferred that the fluoromonomer used in this process is tetrafluoroethylene. TFE polymerizes to form PTFE.
Substrates specifically exemplified for the present invention include wood, molded polyimide parts, porous polyimide powder, porous para-aramids such as poly(para-phenylene terephthalamide) [PPD-T] in the forms of powder, pulp and/or fiber, and porous meta-aramids, such as poly(m-phenylene isophthalamide)[MPD-I] in the forms of powder, fibers or fibrids, porous polyurethane, and leather (pigskin and cowskin).
In the case of liquid fluoromonomer, such as PDD and PMD, the carrier solvent can be the monomer or the monomer containing a small amount of initiator solution (for example, DP in a Freon(copyright) solvent).